struct TrafficLight {
    color: String,
}

impl TrafficLight {
    // 使用 `Self` 填空
    pub fn show_state(self: &Self) {
        println!("the current state is {}", self.color);
    }

    // 填空，不要使用 `Self` 或其变体
    pub fn change_state(&mut self) {
        self.color = "green".to_string()
    }
}
fn main() {}
